1. Field of the Invention
The present invention relates to an acousto-optic tunable filter that is used in various optical apparatus such as spectroscopes and spectrophotometers and more particularly relates to a method of calculating the equivalence incident angle.
2. Description of the Related Art
In general, spectroscopes, spectrophotometers and the like are used in spectroscopy that obtains a spectrum by measuring the intensity of a source light beam in different wavelength regions. Prism spectroscopes and diffraction grating spectroscopes have been widely used as such spectroscopes. However, the acousto-optic tunable filter, abbreviated as AOTF, has spread recently, because of its high-speed and aseismic processing. In the AOTF, an acoustic wave is applied to an acoustic medium consisting in a uniaxial crystal such as a tellurium dioxide (TeO.sub.2) crystal. At the same time, a source light beam is radiated onto the acoustic medium to obtain a particular wavelength component of the source light beam as a diffracted ray diffracted within the acoustic medium. Here, the wavelength of the diffracted light is determined by the frequency of the applied acoustic wave, so that the spectrum of the source light beam is obtained by varying the frequency of the acoustic wave and by continuously measuring the intensity of the diffracted ray using a photometer.
The history of the development and advances of the AOTF in recent years is as follows. In 1967, an AOTF of collinear type was first realized for practical use in 1967. Here, the direction in which the acoustic wave travels is the same as the direction in which the light beam travels. However, the most practical and useful AOTFs were not realized until I. C. Chang discovered that TeO.sub.2 is an almost ideal crystal material for manufacturing the AOTF and a non-collinear type AOTF was proposed. In the non-collinear type AOTF, the direction in which the acoustic wave travels intersects with the direction in which the light beam travels. During the past 20 years, hundreds of patents and papers have been disclosed, but almost all of these research and development works are based on early theoretical contributions of I. C. Chang, T. Yano, and A. Watanabe, in which momentum matching and phase matching conditions are commonly accepted.
In the early theoretical research and development, physical models for the AOTF were perfect, but the mathematical analysis always depended on approximation methods. In 1985, Mo Fuqin proposed, for the first time, an accurate mathematical description about the parallel-tangent condition. In 1987, Epikhin gave a general system of equations that represent accurate relationships between acoustic parameters and optical parameters. This is one of the greatest contributions to AOTF designing. In 1991, Gass set the acoustic wavevector angle at -80.23.degree. for no particular reason to calculate optimal parameters for the system. In 1992, Ren Quan almost entirely followed the analytic method of Gass's paper and set the acoustic wavevector angle at 105.degree. to calculate a set of parameters for this particular acoustic wavevector angle. However, optimal acoustic wavevector angles have not been given through general study for an entire perfect-phase-matching curve.
As described above, the AOTF has rapidly spread and progressed of recent years, but a number of problems to be solved remain with prior AOTFs. One of the problems is as follows. In the AOTF in general, two diffracted rays, a diffracted ordinary ray of wavelength .lambda..sub.i and a diffracted extraordinary ray of wavelength .lambda..sub.i ' are obtained from a source light beam that contains an incident extraordinary ray and an incident ordinary ray, through diffraction within the acoustic medium, depending on the wavelength of the acoustic wave (See FIG. 1). Here, the-wavelengths .lambda..sub.i and .lambda..sub.i ' are relatively close. A prior AOTF used one of the diffracted rays, usually the diffracted ordinary ray to obtain the spectrum of the source light beam.
However, in the prior AOTF, if the intensity of the source light beam is low, or if the intensity of the components of the source light beam in a wavelength region is low, then the accuracy of spectrometry or the accuracy of the finally obtained spectrum becomes low. For example, in the spectral analysis of measuring the absorption spectrum of an object, it is required to accurately measure the intensity of the light in a wavelength region that is absorbed by the object, but the light intensity is often low in the wavelength region owing to the absorption. Therefore, there has been a problem that the accuracy of the spectral analysis becomes low if an AOTF is used.